| Abstract/Description or Keywords |
Hydrocarbon pipelines are currently proposed to cross west central British Columbia (B.C.) to access a deepwater port at Kitimat. The geology and geomorphology of the area is complex and destructive landslides are common. The northwest trending rugged topography poses serious challenges for pipeline development. Only certain valleys and passes are suitable for east–west oriented infrastructure. The terrain across west central B.C., with steep unstable rock masses and weak soils, places considerable constraints on pipeline development. This paper provides an overview of the landscape, terrain, hillslope processes and fluvial processes found within the general area of the proposed pipeline corridor across west central B.C. The intent of this paper is to help formulate discussion, encourage more in-depth study, direct more detailed on-the-ground investigation, and stimulate investigation into possible safer alternative routes to the unstable terrain found in west central B.C. This paper does not discuss environmental consequences and risk associated with the proposed pipelines although the environmental consequences of an oil pipeline break do differ considerably from a break sustained by a natural gas pipeline. The proposed corridor crosses three distinct physiographic units: the Nechako Plateau, the Hazelton Mountains, and the Kitimat Ranges. These units are distinct topographically as reflected in present day landforms, erosion, and landslides, and thus present different hazards to a pipeline. The Nechako Plateau appears relatively benign; however, large landslides have occurred in volcanic rock overlying other older volcanic and sedimentary rock. Active bedrock spread is occurring to the east of Parrott Creek, possibly foreshadowing further movement along the northwest-southeast trending ridges running between Houston and Francois Lake. Along the Morice River, advance-phase glaciolacustrine sediments have historically experienced landslides. Road construction and wildfires have reactivated these landslides. The proposed pipeline corridor crosses an historic earth flow west of Owen Creek, glaciolacustrine sediment along Owen Creek, and probably buried advance-phase glaciolacustrine sediments near Owen Creek, Fenton Creek and Lamprey Creek. The pipeline corridor follows the Crystal forest access road up Gosnell Creek. Shifting channels on active alluvial fans pose road maintenance challenges along a 10 km section of the road. Pipelines will likely present similar challenges crossing these fans. There is considerable lateral bank instability at the proposed Crystal Creek and Gosnell Creek crossing. The proposed pipeline corridor dissects the floodplain (glaciofluvial and glaciolacustrine terrain) located immediately upstream from the Clore Canyon. No development of any sort has occurred to date upstream of the Clore Canyon so it is unclear how this terrain will respond to pipeline development. The volcanic bedrock of the Hazelton Mountains is inherently unstable as evident in many prehistoric landslides. Three documented large landslides within the Bulkley Range of the Hazelton Mountains have severed the natural gas pipeline since its construction in the early 1970s; large landslides have also impacted forest roads and highways. Deep-seated gravitational slope deformation is prevalent in the volcanic bedrock found in the Kitnayakwa, Clore and Bernie watersheds. Sackungen or slope sagging, indicative of slope deformation, indicates active slope movement that commonly foreshadows a pending landslide. A thorough geotechnical investigation is required to determine the stability of the bedrock and hillslope in areas of slope deformation. Avoidance of these unstable hillslopes is generally the preferred engineering development option. The proposed corridor crosses through a mountainside to the southeast of the Clore Canyon. The highly fractured bedrock in the canyon is undergoing active mass erosion. This visibly (A40500) unstable rock reaches up to about 1200 m above sea level (asl) and extends around the mountain into an adjacent tributary valley. This bedrock along the north and west side of the mountain is extensively gullied and contains many landslide scarps and an actively moving landslide. Along the east side of the mountain, sackungen parallel the slope and extend through old landslide scarps. The active instability of the mountain slope places major constraints on development. Steep narrow valleys characterize the Kitimat Ranges. Colluvial-fluvial fans are at the base of most steep gully channels in the Hoult Creek and Upper Kitimat watershed. These steep gully channels extend from the alpine onto the valley flat or directly into Hoult Creek or the Kitimat River. Many of these high-energy systems experienced debris flows during extreme rainstorms in the fall of 1978 and the fall of 1992. Debris flows commonly occur under seemingly _normal_ storm events during summer convective storms and fall frontal rainstorms. Debris flows are powerful landslides that can damage or rupture pipelines. Hunter Creek, a large active alluvial fan, has historically pushed the Kitimat River across the valley. The most recent catastrophic channel avulsion occurred in 1992. This avulsion was caused by road construction up the fan and the construction of a levée above the bridge crossing on Hunter Creek. Channel changes will likely recur on the fan during major flood events. The Kitimat Trough, situated between Terrace and Kitimat, is an uplifted fiord. Sensitive glaciomarine sediments occupy much of the valley floor. Deep deposits of glaciofluvial sediments and postglacial materials (floodplain, alluvial fans and bogs) cover the glaciomarine sediments. These glaciomarine sediments have experienced large prehistoric and recent landslides. A high incidence of prehistoric landslides occur around Mink Creek, the Nalbeelah wet land complex, the foreslope of the Onion Lake Flats (fan-delta), Cecil Creek and Deception Creek. Recent large flow slides occurred at Mink Creek (winter 1992-93) and Lakelse Lake in May and June 1962. A large submarine flow slide occurred in sensitive marine muds at the front of the fiord-head delta at Kitimat Arm in April 1975. These recent landslides serve to show the continuing sensitivity of the glaciomarine sediments in the Kitimat Trough and the marine sediments on the fan-delta at the fiord-head of Kitimat Arm. Natural and human caused factors such as increases in surface load, removal of lateral support by stream bank undercutting or excavation, vibration by heavy equipment, earthquake shock, high water pressures and interruption of intertidal drainage can trigger these landslides. Thus, the potential exists for landslides to occur during pipeline construction and in the future. Glaciomarine sediments in the vicinity of Cecil Creek, Deception Creek, Wedeene River, Little Wedeene River, along the west side of Kitimat Arm and along Chist Creek will be encountered during pipeline construction. The pipeline corridor crosses features indicative of prehistoric flow slides near Cecil Creek through to the little Wedeene River. The presence of prehistoric flow slides in the glaciomarine sediments suggest a high probability that future landslides will occur. These failures commonly start with a small landslide from bank erosion or loading that exposes a layer of sensitive material. Then, they rapidly retrogress with a flow of material from the displacement basin. Pipelines crossing glaciomarine sediments must therefore avoid areas that lie within potential flow slide depletion zones as landslides will break or disrupt pipeline service. Landslides travel long distances and damage linear infrastructure such as pipelines. Six large rock slides occurred in west central B.C. since 1978, five of these since 1999, and four since 2002. Three of the six rock slides severed the natural gas pipeline (Howson landslides in 1978 and 1999, and Zymoetz landslide in 2002). Damage to linear infrastructure commonly occurs in runout zones many kilometres from the initial landslide. This has occurred with recent landslides in west central B.C.; the longest traveled in excess of 4 km along a slope of 9°. Therefore, the potential for damage to pipelines ext ends to unstable terrain and potential landslides that start well outside the construction corridor. Long periods of increasing precipitation and temperature are associated with most dated, large landslides across northern B.C. The climate of northern B.C. appears to have become warmer and wetter since the beginning of instrumental observations. There is evidence to suggest that landslide rates have increased in west central B.C. Climate change scenarios suggest a warmer and wetter climate for west central B.C. Therefore, the rate of landslide occurrence will likely increase and thus the likelihood of landslide impact to a pipeline will increase. Recognition and avoidance of unstable terrain is the most efficient and cost effective method for management in landslide prone terrain. This requires detailed terrain stability mapping and geotechnical investigation to identify unstable slopes, runout zones, and depletion zones. Avoidance of unstable terrain is a difficult management strategy to adopt over many sections of the proposed pipeline corridor given the topographic constraints. Therefore, the unstable mountainous terrain across west central B.C. is not a safe location for pipelines. Eventually a landslide will sever a pipeline. An alternative safer route through B.C. needs investigation. |
